Heat is often created as a byproduct of industrial processes where flowing streams of high-temperature liquids, solids, or gases must be exhausted into the environment or removed in some way in an effort to maintain the operating temperatures of the industrial process equipment. Sometimes the industrial process can use heat exchanger devices to capture the heat and recycle it back into the process via other process streams. Other times it is not feasible to capture and recycle this heat either because its temperature is too high or it may contain insufficient mass flow. This heat is referred to as “waste” heat and is typically discharged directly into the environment or indirectly through a cooling medium, such as water or air.
This waste heat can be converted into useful work by a variety of turbine generator systems that employ well-known thermodynamic methods, such as the Rankine cycle. These thermodynamic methods are typically steam-based processes where the waste heat is recovered and used to generate steam from water in a boiler in order to drive a corresponding turbine. Organic Rankine cycles replace the water with a lower boiling-point working fluid, such as a light hydrocarbon like propane or butane, or a HCFC (e.g., R245fa) fluid. More recently, and in view of issues such as thermal instability, toxicity, or flammability of the lower boiling-point working fluids, some thermodynamic cycles have been modified to circulate more greenhouse-friendly and/or neutral working fluids, such as carbon dioxide or ammonia.
A pump is required to pressurize and circulate the working fluid throughout the working fluid circuit. The pump is typically a motor-driven pump, however, these pumps require costly shaft seals to prevent working fluid leakage and often require the implementation of a gearbox and a variable frequency drive which add to the overall cost and complexity of the system. Replacing the motor-driven pump with a turbopump eliminates one or more of these issues, but at the same time introduces problems of starting and “bootstrapping” the turbopump, which relies heavily on the circulation of heated working fluid for proper operation. Unless the turbopump is provided with a successful start sequence, the turbopump will not be able to bootstrap itself and thereafter attain steady-state operation.
What is needed, therefore, is a system and method of operating a waste heat recovery thermodynamic cycle that provides a successful start sequence adapted to start a turbopump and bring it to steady-state operation.